Simplify tagging queries with a view that uses recursive CTEs

[bradenmacdonald]

This is a proposed fix for openedx/modular-learning#257 .

Approach: It uses recursive common table expressions to update the API to support taxonomies of unlimited depth. This is mostly an implementation detail, and the API does not change except for returning more results in the case where it was previously limited to a depth of 3.

Actual depth limit: This PR also clarifies the definition of TAXONOMY_MAX_DEPTH and actually enforces it to limit the allowed depth to six levels. (Before this, no limit was enforced when creating tags! A limit of 3 levels was enforced when reading tags from multiple levels at once, but it didn't work well below the root.)

Perfomance: This implementation is ever so slightly slower, but it's not significant. Compare it's test run (4m43s) with a recent master test run (4m34s). If we need to improve performance in the future, a path to doing so is outlined in the TagComputed docstring, although it would require a more significant refactor of the implementation.

AI disclosure: Claude wrote the CTE queries and helped with some of the refactoring.

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